Method and device for separating heavy particles from a particulate material

ABSTRACT

In a method for separating heavy particles from a particulate material containing such heavy particles, use is made of a device having a Coanda sifter, which is adapted to receive a flow of the particulate material and which has a downwardly-facing, convexly single-curved surface (9), and a zigzag sifter (13) arranged after the Coanda sifter and having a plurality of zig-zag-shaped vertical walls (14) which are perpendicular to the axis of curvature of the single-curved surface (9) and define between them channels (15) extending upwards in zig-zag fashion. The particulate material is transported to the Coanda sifter and is conducted over its convexly single-curved surface (9) to have imparted to it a direction of motion (P1, P2) which is dependent on the weight of the particles and in which the particles are guided into the zigzag sifter (13). The particles are guided into the channels (15) of the zigzag sifter (13) in the lower portions thereof and from the side parallel to the channel walls (14). The particles are transported up through the channels (15) with the aid of a flow of transport air which is so adjusted that the heavy particles are separated in the zigzag sifter. (13).

BACKGROUND AND SUMMARY OF THE INVENTION

The present invention relates to a method for separating heavyparticles, such as latex particles, from a particulate material, such aswood fibres produced from rubber trees, which contains such heavyparticles and which is transported by means of air from a first station,such as a container, to a second station, such as a plant formanufacturing fibreboards. The invention also relates to a device forcarrying out the method.

In the manufacture of fibreboards, wood fibres are pneumaticallytransported, first from a drying plant to a collecting container andthence to a forming station. In the forming station, the fibres areplaced as a mat on a wire, through which the transport air is suckedoff. The fibre mat is thereafter pressed into fibreboards under givenpressure and temperature conditions.

In the manufacture of fibreboards, it is essential that contaminants inthe form of heavy particles be separated from the flow of material. In amethod, known from PCT/SE91/00603 (SE 9003092-5), for separatinglarge-size and hence heavy particles from the wood fibres whentransported from the collecting container to the forming station, thewood fibres are introduced into a settling chamber as adownwardly-directed flow of material via a downwardly-facing settlingchamber inlet, and is transported away from the settling cheer as anupwardly-directed flow of air-material via a settling chamber outletdisposed beside the inlet. Jets of compressed air, directed obliquely atthe downwardly-directed flow of material, are blown into the settlingchamber in order to decelerate the flow of material and to spread it inthe transverse direction towards the outlet, the flow of transport airbeing blown into the settling chamber so as to encounter from below thedecelerated and spread flow of material and, together with it, form theabove-mentioned flow of air-material.

While this known method has proved to serve its purpose quite well forseparating heavy particles in the form of glue lumps and fibre flocksfrom a particulate material in the form of glue-coated wood fibres, ityields an unsatisfactory result when separating heavy particles in theform of latex particles from a particulate material in the form of woodfibres produced from rubber trees.

One object of the present invention is to provide a method making itpossible to efficiently separate also latex particles from wood fibresproduced from rubber trees.

According to the invention, this object is achieved by a method which isof the type stated by way of introduction and which is characterised inthat the particulate material in a substantially downwardly-directedflow is transported in known manner to a so-called Coanda sifter, inwhich the material is conducted over a downwardly-facing, convexlysingle-curved surface in order to impart to the particles a direction ofmotion which is dependent on the weight of the particles and in whichthe particles are guided into a so-called zigzag sifter disposed afterthe Coanda sifter and having a plurality of zigzag-shaped, substantiallyvertical walls which define between them channels extending upwards inzigzag fashion, the particles being guided into the channels of thezigzag sifter in the lower portions thereof and from the sidesubstantially parallel to the walls, and that the particles aretransported in known manner up through the zigzag-shaped channels bymeans of a flow of transport air which is so adjusted that the heavyparticles are separated in the zigzag sifter.

Another object of the present invention is to provide a device forcarrying out this method.

According to the invention, this object is achieved by means of a devicewhich is characterised by a so-called Coanda sifter which is adapted inknown manner to receive a substantially downwardly-directed flow of theparticulate material and which has a downwardly-facing, convexlysingle-curved surface in order to impart To the particles a direction ofmotion dependent on the weight of the particles, and a so-called zigzagsifter disposed after The Coanda sifter and having a plurality ofzigzag-shaped, substantially vertical walls which are substantiallyperpendicular to the axis of curvature of the single-curved surface anddefine between them channels which extend upwards in zigzag fashion,said zigzag sifter being adapted to receive the particles coming fromthe Coanda sifter, in its lower portion and from the side.

The folds defining the zigzag shape of the walls are inclined preferably5°-20°, especially 15°, downwards in relation to the horizontal plane ina direction away from the Coanda sifter.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in more detail with reference to theaccompanying drawings.

FIG. 1 schematically shows a device for separating heavy particles froma particulate material.

FIG. 2 shows a zigzag sifter included in the device of FIG. 1 in thedirection of the arrow II in FIG. 1.

DETAILED DESCRIPTION

The device schematically illustrated in the drawings is arranged betweena container 1, which holds wood fibres derived from rubber trees andcontaining contaminants in the form of heavy particles, such as latexparticles, and a forming station (not shown), where fibreboards areformed.

The bottom of the container 1 consists of the upper run of an endlessconveyor belt 2, on which a heap 3 of wood fibres is supported. Aplurality of rotatable slushing rollers 4, provided with spikes orteeth, are arranged in the front part of the container 1 in order, asthe conveyor belt 2 is discharging wood fibres from the container 1, tobreak up fibre flocks and lumps which have formed when storing thefibres in the container 1.

The container 1 has an outlet 5 which is located at the exit end of theconveyor belt 2 and communicates with a substantially vertical dischargeshaft 6. The container 1 has a front wall 7 forming an extension of thefront wall 8 of the shaft 6 and having an air intake 7' providedtherein.

The front wall 8 of the shaft 6 passes at its lower portion into adownwardly-facing, convexly single-curved surface 9 which in theillustrated embodiment consists of the lower half of the outercircumferential surface of a horizontal, straight, circular cylinder.The surface 9 forms a so-called Coanda sifter known in the art, i.e. asifter relying on the Coanda effect. A sifter of this type is describedin more detail in Swedish Patent Specification 8505726-3. The rear wall10 of the shaft 6 has in its lower portion, on a level with the Coandasifter, an inlet opening 11. A fan 12 is adapted, via the opening 11, toblow a high-velocity air flow directed substantially at the surface 9into the lower part of the shaft 6.

The wood fibres fed from the container 1 into the shaft 6 by means ofthe conveyor belt 2 are transported with the aid of the air flow towardsthe Coanda sifter. As a result of the Coanda effect, the air flow willfollow the curved surface 9 and entrain the wood fibres (arrows P1 inFIG. 1), while the heavy particles are not deflected to the same extent,but follow a straighter path (arrows P2 in FIG. 1).

A so-called zigzag sifter 13 known per se is arranged immediately afterthe Coanda sifter for separating the heavy particles (latex particles,glue lumps etc). A zigzag sifter of this type is described in GermanOffenlegungsschrift 1,482,424. The zigzag sifter 13 has a plurality ofzigzag-shaped, vertical metal sheets 14 (FIG. 2), which areperpendicular to the axis of curvature of the surface 9 and definebetween them channels 15 extending upwards in zigzag fashion. As shownin FIG. 1, the wood fibres (arrows P1) and the heavy particles (arrowsP2) leaving the Coanda sifter are guided into the channels 15 of thezigzag sifter 13 in the lower portions thereof and from the side, i.e.parallel to the sheets 14. Then, the wood fibres and the heavy particlesare distributed when entering the zigzag sifter 13 across the width ofthe channels 15, such that the heavy particles will enter deeper (to theleft in FIG. 1) than the wood fibres. An additional distribution of thewood fibres and the heavy particles, resulting in a deflection of theheavy particles to the left (FIG. 1) during the operation of the zigzagsifter 13, is obtained by the zigzag sifter being so inclined that thefolds 14' defining the zigzag shape of the sheets 14 are inclined 5°-20°downwards in relation to the horizontal plane in a direction away fromthe Coanda sifter. In the illustrated embodiment, this inclination isabout 15°.

A conveyor screw 16 is arranged in the lower, left-hand part (FIG. 1) ofthe zigzag sifter 13 for removing the heavy particles separated in thezigzag sifter 13. An air supply conduit 17 opens in the lower,right-hand part (FIG. 1) of the zigzag sifter 13. A fan 18 is provided,via the conduit 17, to blow a flow of transport air into the zigzagsifter 13. This flow transports the wood fibres through the zigzagsifter while the heavy particles (latex particles) are separated thereinand drop on to the conveyor screw 16.

The zigzag sifter 13 has an upper outlet 19 having an upwardlydecreasing cross-sectional area and communicating with a transportconduit 20 for transporting the wood fibres freed from heavy particlesto the forming station. In its upper portion, the transport conduit 20has a 180° bend, after which it is divided into two separate conduits20a and 20b, of which the conduit 20a passes on to the forming stationand the conduit 20b returns to the zigzag sifter 13 and opens at themouth of the conduit 17. Guide baffles 21 are arranged in the lower partof the conduit 20b.

The flow of air-material consisting of transport air and wood fibres isaccelerated in the outlet 19 as a result of the decreasingcross-sectional area thereof. The flow then has a high velocity in thebend of the transport conduit 20, causing the wood fibres therein to beflung outwards so as to follow the outer wall of the bend in order,together with a portion of the transport air, to be guided into theconduit 20a, leading to the forming station. The rest of the transportair is guided into the conduit 20b and recycled to the zigzag sifter.

The device described above yields a good result also in the separationof latex particles from wood fibres derived from rubber trees. This isassumed in particular to depend on the distribution of the particlesacross the width of the channels 15 which is brought about with the aidof the Coanda sifter and to which the inclination of the zigzag sifter13 also contributes.

Since the particles are spread over the width of the zigzag sifter 13,such that the particles will enter deeper into the sifter (i.e. fartherto the left in FIG. 1) the heavier they are, the wood fibres freed fromheavy particles may, if so desired, easily be divided, for example intotwo fractions. This is done by dividing the upper outlet 19 of thezigzag sifter 13 into two separate outlets, one disposed over the upper,left-hand part (FIG. 1) of the zigzag sifter 13, and the other over theupper, right-hand part (FIG. 1) thereof.

I claim:
 1. A method for separating heavy particles from a particulatematerial which contains the heavy particles, comprising the stepsof:transporting the particulate material in a substantiallydownwardly-directed flow to a Coanda sifter; conducting the particulatematerial, in the Coanda sifter, over a downwardly-facing, convexlysingle-curved surface such that a direction of motion is imparted toparticles of the particulate material, the direction of motion of theparticles being dependent on a weight of the particles; guiding theparticles into a zigzag sifter disposed after the Coanda sifter, thezigzag sifter having a plurality of zigzag-shaped, substantiallyvertical walls, the walls defining channels between them, the channelsextending upwards in zigzag fashion; guiding the particles into thechannels of the zigzag sifter in a lower portion of the zigzag sifterand from a side of the zigzag sifter, the side being substantiallyparallel to the walls; transporting the particles up through thezigzag-shaped channels by means of a flow of transport air; and settingthe flow of transport air such that the heavy particles are separated inthe zigzag sifter.
 2. A device for separating heavy particles from aparticulate material, the particulate material containing the heavyparticles comprising:a first station for containing a supply ofparticulate material containing heavy particles; a second station forreceiving particulate material from which heavy particles have beenremoved; a Coanda sifter; means for directing a substantiallydownwardly-directed flow of the particulate material from the firststation to the Coanda sifter; the Coanda sifter having adownwardly-facing, convexly single-curved surface for imparting adirection of motion to particles of the particulate material, thedirection of motion being dependent on a weight of the particles; azigzag sifter disposed between the Coanda sifter and the second station,the zigzag sifter having a plurality of zigzag-shaped, substantiallyvertical walls which are substantially perpendicular to an axis ofcurvature of the single-curved surface the walls defining channelsbetween them, the channels extending upwards in zigzag fashion, thezigzag sifter being arranged relative to the Coanda sifter such that thezigzag sifter is adapted to receive particles from the Coanda sifter ina lower portion of the zigzag sifter and in a side of the zigzag sifter.3. A device as claimed in claim 2, wherein the walls have a zigzagshape, the zigzag shape being defined by folds, the folds being inclined5°-20° downwards in relation to a horizontal plane and in a directionaway from the Coanda sifter.
 4. The device as claimed in claim 2,further comprising means for forcing a flow of air upwards through thezigzag sifter.
 5. The device as claimed in claim 4, further comprisingmeans for adjusting a flow rate of the flow of air.
 6. The device asclaimed in claim 2, wherein the first station is a container.
 7. Thedevice as claimed in claim 2, wherein the second station is a plant formaking fiberboard.